Virginia Grape Disease Updates

As we expected, Winchester area had a wet weekend.  We had to major rain events, which resulted in 11 hours and 10 hours of wetness, both are in upper 60F to lower 70F range.  These are downy mildew and Botrytis risk events.  Once again, humid nights we are experience can be a precursor event for downy mildew risk events because these nights favors spore production of downy mildew pathogen.

Winchester area received a shower yesterday, resulted in 5+ hours of wetness in lower 70F.  It was an infection risk event for downy mildew, and very low infection risk event for Botrytis.  In addition, we observed dews in the last two nights, indicating high humidity, which can trigger downy mildew to produce spores.  As usual, late season downy tends to appear on younger leaves on the top of the canopy.  Please keep eyes on them!  There is a chance of rain on Sunday and Monday.

Winchester area had a wet weekend.  We ended up having 9+ and 16+ hours of wet events, and both of them are in upper 60F.  Needless to say, these were disease risk events for downy mildew and Botrytis.  These were events for black rot too, but the clusters should be resistant to infection at this point.  Looks like we are expecting some rain during this week too.  Please keep your eyes on downy mildew.

After 13+ hours of wetness on the 13th-14th rain event, we had another wetness event starting from last night.  It resulted in 11 hours of wetness with an average temperature near 70F.  It was infection event for downy mildew, black rot, Phomopsis and Botrytis.  With the back to back rain event and high humidity during the night, I would keep eyes on downy mildew development.  It tends to show up on younger leaves (at the top of the canopy) first.  Also, many of varieties are about to be berry-touch/bunch closure stage, which is one of timings for Botrytis application.

Winchester area received a shower yesterday (11th, very early in the morning) which resulted in ~7 hours of wetness with an average temperature of high 60F.  It was risk event for downy mildew (with a good chance of spore production because it was a night time event) and Phomopsis, and a borderline case for Black rot.  (once again, please note that your berries should have become more resistant to downy, powdery, and black rot infection at this point).  Since we had a back-to-back rain event, please keep eye on next potential rain for downy mildew, especially you are due to a next application.  There are chances of rain on Monday and Tuesday.

A thunderstorm went through Winchester area yesterday evening, resulted in 12+ hours of wetness (on going) with an average temperature of near 70F.  This most likely to be resulted in risk event for downy mildew, black rot and Botrytis. However, please note that the risk of black rot (especially on clusters) should be low because vines are about to be done (or already done) with their critical period for cluster infection.  (Plus, if you have not seen much black rot until this point, the risk is low too.)  The same goes with downy mildew cluster infection, but downy mildew can still infect leaves, especially the young ones. People in the south and some early varieties in the north is approaching bunch closure.  This is one of application timing for Botrytis so that we can get the material to the inside of the clusters.

Finally, we are having relatively dry weeks!  Most of us are about to finish critical time where clusters are susceptible to infection by downy mildew, powdery mildew, and black rot.   This critical time varies by varieties, but in general 4 to 6 weeks and 3-4 weeks from bloom for V. vinifera and V. labrusca species, respectively.   After this critical period, you should be able to relax a bit because these pathogens no longer able to cause disease on berries. So, what’s next?   As usual, disease dynamics really depends on environmental conditions, varieties grown, and cultural practice you employ, but in general, this is the moment when you will be thinking about late season diseases such as Botrytis, ripe rot, bitter rot, and sour rot.   The spray timing for Botrytis is at bloom, bunch closure, and veraison.   The pathogen seems to be active throughout the season, and the reason we recommend application of a Botrytis specific material at bunch closure